Rotary machines, such as steam and gas turbines, used for power generation and mechanical drive applications are generally large machines consisting of multiple turbine stages. In turbines, high pressure fluid flowing through the turbine stages must pass through a series of stationary and rotating components, and seals between the stationary and rotating components are used to control leakage. The efficiency of the turbine is directly dependent on the ability of the seals to prevent leakage, e.g., between the rotor and stator. Turbine designs are conventionally classified as either impulse, with the majority of the pressure drop occurring across fixed nozzles, or reaction, with the pressure drop more evenly distributed between the rotating and stationary vanes. Both designs employ rigid tooth, i.e., labyrinth, seals to control leakage. Traditionally, rigid labyrinth seals of either a hi-lo or straight shaft design are used. These types of seals are employed at virtually all turbine locations where leakage between rotating and stationary components must be controlled. This includes interstage shaft seals, rotor end seals, and bucket (or blade) tip seals. Steam turbines of both impulse and reaction designs typically employ rigid, sharp teeth for rotor/stator sealing. While labyrinth seals have proved to be quite reliable, their performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a "mushroom" profile and opening the seal clearance.
Another type of seal used in many environments, including rotary machines, is a brush seal. Brush seals are generally less prone to leakage than labyrinth seals. A brush seal can also accommodate relative radial movement between fixed and rotational components, for example, between a rotor and a stator, because of the flexure of the seal bristles. Brush seals also generally conform better to surface non-uniformities. The result of using brush seals is better sustained rotary machine performance than is generally possible with labyrinth seals.